Calcium Additions and Microbial Nitrogen Cycle Processes in a Northern Hardwood Forest Peter M. Groffman, 1 * Melany C. Fisk, 2 Charles T. Driscoll, 3 Gene E. Likens, 1 Timothy J. Fahey, 4 Christopher Eagar, 5 and Linda H. Pardo 6 1 Institute of Ecosystem Studies, Box AB, Millbrook, New York 12545, USA; 2 Department of Biology, Appalachian State University, Boone, North Carolina 28608, USA; 3 Department of Civil and Environmental Engineering, Syracuse University, Syracuse, New York 13244, USA; 4 Department of Natural Resources, Cornell University, Ithaca, New York 14853, USA; 5 Northeastern Research Station, USDA Forest Service, PO Box 640, Durham, New Hampshire 03824, USA; 6 Northeastern Research Station, USDA Forest Service, PO Box 968, Burlington, Vermont 05402, USA ABSTRACT Evaluating, and possibly ameliorating, the effects of base cation depletion in forest soils caused by acid deposition is an important topic in the northeastern United States. We added 850 kg Ca ha )1 as wol- lastonite (CaSiO 3 ) to an 11.8-ha watershed at the Hubbard Brook Experimental Forest (HBEF), a northern hardwood forest in New Hampshire, USA, in fall 1999 to replace calcium (Ca) leached from the ecosystem by acid deposition over the past 6 decades. Soil microbial biomass carbon (C) and nitrogen (N) concentrations, gross and potential net N mineralization and nitrification rates, soil solution and stream chemistry, soil:atmosphere trace gas (CO 2 ,N 2 O, CH 4 ) fluxes, and foliar N concentrations have been monitored in the treated watershed and in reference areas at the HBEF before and since the Ca addition. We expected that rates of microbial C and N cycle processes would increase in response to the treatment. By 2000, soil pH was increased by a full unit in the Oie soil horizon, and by 2002 it was increased by nearly 0.5 units in the Oa soil horizon. However, there were declines in the N content of the microbial biomass, potential net and gross N mineralization rates, and soil inorganic N pools in the Oie horizon of the treated watershed. Stream, soil solution, and foliar concentrations of N showed no response to treat- ment. The lack of stimulation of N cycling by Ca addition suggests that microbes may not be stimu- lated by increased pH and Ca levels in the naturally acidic soils at the HBEF, or that other factors (for example, phosphorus, or Ca binding of labile or- ganic matter) may constrain the capacity of mi- crobes to respond to increased pH in the treated watershed. Possible fates for the approximately 10 kg N ha )1 decline in microbial and soil inorganic pools include components of the plant community that we did not measure (for example, seedlings, understory shrubs), increased fluxes of N 2 and/or N storage in soil organic matter. These results raise questions about the factors regulating microbial biomass and activity in northern hardwood forests that should be considered in the context of pro- posals to mitigate the depletion of nutrient cations in soil. Key words: acid deposition; calcium; carbon; Hubbard Brook; microbial biomass; nitrification; nitrogen; phosphorus. Received 23 November 2005; accepted 10 April 2006; published online 19 January 2007. *Corresponding author; e-mail: groffmanp@ecostudies.org Ecosystems (2006) 9: 1289–1305 DOI: 10.1007/s10021-006-0177-z 1289